The distance between a camera and a spatial point in a scene can be determined or well estimated from the position of the point within two or more associated images showing the same point, wherein the associated images are captured simultaneously. The distance calculation is still possible if one or more mirrors are arranged in the scene, and some of the images are captured in the mirror. The three dimensional (3D) position of a point can be computed from basic geometric relationships when the spatial relationship between the image recording device and the position and the parameters of the reflecting surfaces (e.g. mirrors) are known. The challenge in computing an unknown distance from multiple images using reflecting surfaces is called catadioptric stereo vision. In J. Gluckman and S. K. Nayar: Catadioptric Stereo Using Planar Mirrors (International Journal on Computer Vision, 44(1), pp. 65-79, August 2001), the basic theory of catadioptric stereo image generation is described. In such a process, only one camera and a flat mirror with a known position relative to the camera are used.
The U.S. Pat. No. 8,189,100 discloses a portable device comprising a first image sensor, a second image sensor configured to change position with respect to the first image sensor, a controller configured to control the position of the second image sensor, and an image processing module configured to process and combine images captured by the first and second image sensors. Although this device is equipped with two image sensors to produce, for example, a stereo image, both of the image sensors directly capture an image of the real object, and no external mirror is used in the image generation process.
Since nowadays most of the portable communication or computing devices, such as mobile phones or tablets, are usually equipped with two cameras, typically on their opposite sides, i.e. a front camera and a rear camera, there is a need of using such devices to produce a depth image for a particular object.